This invention relates to the field of display systems, more particularly to color display systems that use a falling raster or scrolling color scan.
Modem projection light valve-based display systems typically use one of two schemes to produce full color images. Some systems use three light valves, each producing a primary color image. The three primary colored images are superimposed to give the viewer the perception of a full-color image. Three light valve display systems are generally expensive since there are essentially three separate projection systems and a set of dichroic filters uses as color splitters and combiners.
An alternative to the three light valve color systems uses only one light valve. The single light valve color systems sequentially produce three primary color images that are displayed in a rapid sequence. The viewer""s eye integrates the three images giving the impression of a single full-color image. Single light valve color displays are typically less expensive than the three light valve color displays. However, the single light valve color displays must be very fast in order to produce the three primary colored images in a single frame period.
Micromirror-based display systems use pulse width modulation to produce images that have intermediate intensity shades. The video image data is received as a sequence of n-bit words, each word representing the intensity of a single pixel. The data is converted, through an operation often referred to as xe2x80x9ccorner turning,xe2x80x9d to a series of bit-planes, each comprised of a single bit of common significance for each pixel in the image. The combination of sequential color, pulse width modulation, and corner turning requires a lot of data processing at a very high data rate. The processing hardware required to perform these functions drives up the cost of the display system.
Prior art sequential color display systems use a rapidly rotating color wheel to sequentially filter the light beam. The wheel rotates fast enough that the boundary between color filters sweeps across the face of the modulator very quickly. The modulator is turned off during the filter transition time, or spoke period, when the light reaching the modulator was not a pure primary color. The light from the color wheel is focused onto the color wheel and modulator to avoid creating an image of the color wheel on the modulator.
One method of reducing the cost of a micromirror based display system is described in U.S. patent application No. TI-25123, which teaches a xe2x80x9cthermometer scalexe2x80x9d pulse width modulation scheme that generates a single pulse representative of the analog intensity of a given pixel for each primary color. The thermometer scale PWM methods are optimally coupled with a falling raster, or scrolling color, illumination system. A falling raster illumination system images the color wheel onto the light valve or modulator so that the boundary between the color segments sweeps across the face of the light valve. As the boundary sweeps across the face of the modulator, the data written to the modulator is changed from data representing a first primary color data to data representing a second primary color.
Although falling raster display systems promise to lower the overall cost of display systems, it is difficult to operate the light valve efficiently in the falling raster mode. Existing color wheels cannot provide multiple color bands to the modulator without relatively high inefficiencies at the interface between the primary color segments. What is needed is a method and system of scrolling two or more color segments across the face of the light valve.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention that provides a method and system for producing a falling raster, or scrolling color, display system. One embodiment of the claimed invention provides a color wheel for a projection display system. The color wheel comprises a color wheel hub defining a center of the wheel, at least one dichroic color filter having a first pass band and supported by the hub, at least one dichroic color filter having a second pass band and supported by the hub, and at least one dichroic color filter having a third pass band and supported by the hub. The first, second, and third dichroic filters forming spirals abutting each other at an interface, the interface between two of the dichroic filters having a radius from said center defined by:
r=axcex8
where r is the radius or distance of the interface from said center, a is a constant, and xcex8 defines an arc between the interface and a reference.
Another embodiment of the disclosed invention provides a display system. The display system is comprised of a light source for generating a beam of light along a first light path, a collection optic on the first light path for collecting the beam of light, a color wheel on the first light path for filtering the beam of light, a light valve on said first light path for receiving the filtered beam of light and selectively modulating the filtered beam of light to produce an image bearing beam of light along a second light path, and a projection optical system on the second light path for focusing the image bearing beam of light onto an image plane. The color wheel is comprised of a color wheel hub defining a center of the wheel, at least one dichroic color filter having a first pass band and supported by the hub, at least one dichroic color filter having a second pass band and supported by the hub, and at least one dichroic color filter having a third pass band and supported by said hub. The first, second, and third dichroic filters forming spirals abutting each other at an interface, the interface between two of the dichroic filters having a radius from the center defined by:
r=axcex8
where r is the radius or distance of the interface from the center, a is a constant, and xcex8 defines an arc between the interface and a reference.